Electric power systems of different types, e.g. electric power transmission systems or railway systems, need to provide electric power in a reliable fashion. Therefore such systems may comprise energy storage devices, for example in form of direct current (DC) power sources. The energy storage devices may provide power for evening out fluctuations, shortages, etc. In for example the electric power transmission system, the load varies considerably during, e.g. day and night, i.e. the active power balance and the reactive power balance in the system thus also varies. The result can be unacceptable voltage amplitude variations, a voltage depression, or even a voltage collapse.
A reactive power compensator may be provided with an energy storage system in the form of a DC power source. Both active and reactive power support can thereby be supplied. A STATic VAr COMpensator (STATCOM) with energy storage is denoted a Battery Energy Storage System (BESS). The BESS can be arranged to continuously provide both active and reactive power compensation to control fluctuations under various system conditions and thereby improve the transmission and distribution performance.
FIG. 1 illustrates such a power compensator 1, and in particular a Static synchronous Compensator (STATCOM) comprising a Voltage Source Converter (VSCs) 2. The VSC 2 is on its alternating current (AC) side connected to a load, for example the electric power transmission system, and on its DC side to a capacitor 3 (or bank of capacitors), constituting a DC voltage source. The power compensator 1 can further be provided with a battery energy storage 4, comprising one or more strings of series-connected batteries.
In a load comprising a single phase AC system, the instantaneous power pulsates with twice the line frequency. Such power pulsation is reflected into the DC link current in the single-phase VSC. If the VSC 2 is used for reactive power compensation, the only energy storage element on the DC side might be the DC link capacitor 3 and the DC link current pulsation will be reflected into a DC link capacitor voltage ripple.
If a battery energy storage 4 is connected in parallel with the DC capacitor 3, the ripple current will be shared between the battery energy storage 4 and the DC capacitor 3. Typically the battery energy storage 4 comprises electro-chemical batteries, and since they are normally fairly voltage stiff, a significant amount of ripple goes into the batteries. Such ripple may harm the batteries, by causing resistive heating and shortening their service life.
A known way of combating this is to interface the battery energy storage 4 with a DC/DC converter, e.g. a DC/DC boost converter. A disadvantage of such solution is that the DC/DC converter has to be rated for the full battery voltage, which may render the solution to be rather costly, in particular for high voltage STATCOM/BESS devices.